The present invention relates to tires of the kind having a crown belt and flexible sidewalls, which are intended for vehicles capable of travelling at high or very high speeds, such as touring, sports or competition vehicles.
Tires of this kind generally have a radial carcass comprising one or more plies which extend continuously from one bead of the tire to the other, and a crown belt which consist of at least one pair of plies formed from cables orientated in opposite directions at small angles of the order of 10.degree. to 30.degree., to the longitudinal direction, said belt confining the crown of the carcass over a width approximately equal to the width of the tread surface.
With a construction of this type, the crown of the tire is relatively stiff owing to the fact that it is reinforced both by the bias cord crown plies of the belt and by the underlying portion or portions of the plies of the carcass, whose cords extend transversely, the whole forming a triangulated structure. This stiffness in the crown helps to reduce wear on the tread and the drift resulting from side forces (centrifugal force on bends, sidewinds, camber). On the other hand it has an unfavorable effect on the ride given by the tire on bad roads since it will not allow the crown of the tire to adapt to the irregularities of the surface as well as can an unbelted tire having a conventional cross-ply carcass. Thus, if a tire having a crown belt is to give a good ride, it must have sidewalls which are highly flexible vertically, which is achieved with the radial carcass.
However, the trend for some years has been to use, for high and very high speed vehicles, tires of lower cross-section than in the past, that is tires in which the cross-sectional ratio H/G is between 0.3 and 0.8. There were various reasons for this: to lower the centre of gravity of the vehicle to which the tires were fitted, to provide or retain in the wheels sufficient room to accommodate brake drums or discs, to improve certain characteristics of the behaviour of the tires, such as recovery from bends, or again to increase safety by reducing the distance of collapse in the event of a sudden blow-out.
In these tires of low cross-section, the crown belt is under a relatively greater tension from the inflation pressure, and is thus stiffer and less conductive to a good ride than in tires of high cross-section, with the result only by increasing still further the vertical flexibility of the sidewalls can it be ensured that the tire still gives a good overall ride. However, there is little scope for improvement in this direction, on the one hand due to the small height of the sidewalls, which reduces the amplitude of their flexing, and on the other because of the need to preserve transverse stability to counteract the wobbling or sideways swaying movement which is observed when the sidewalls are too flexible.
It is an object of the invention to minimise or overcome these disadvantages and of thus obtaining belted low cross-section tires for the uses presently concerned which give a better standard of ride and retain good lateral stability: the invention adopts a manner of construction in which the carcass is confined essentially to the sidewalls of the tire, said carcass being interrupted in the central zone of the crown, where the reinforcement is provided essentially by a belt formed from at least one pair of plies made of cords which intersect at small angles.
This construction had already been proposed in the early years of radial tires, with the object of remedying the poor ride caused by triangulated crown reinforcements (see for example French patent specification No. 1,187,693 in the name of Pneumatiques Caoutchouc Manufacture Et Plastiques Kleber-Colombes. This construction was not however adopted by the industry, in particular because, since the tires then involved had a high cross-sectional ratio greater than 0.8 and the belt was under relatively little tension in the circumferential direction, the crown of the carcass adjacent the belt was still necessary and useful to give the belt adequate stiffness when the belt was only formed by oblique cross-plies. When on the other hand what are involved are tires of low cross-section, where the belt is under more tension, calculations and experience show that the part of the carcass underlying the belt is either not stressed transversely or is so little stressed transversely that it contributes substantially nothing to the road-going qualities of the tire.